Automatic darkening and glare reducing liquid crystal mirror

ABSTRACT

An automatic darkening liquid crystal mirror for vehicles, the mirror having a transparent front substrate and a back substrate with a reflective or transflective mirrored coating. The front and back substrates are spaced apart to define a liquid crystal cell between the substrates that is filled with a liquid crystal fluid incorporating a dichroic dye. A conductive thin film is applied onto the interior surface of the front substrate, and the mirrored coating of the back substrate also is conductive. An alignment compound is deposited on opposite sides of and bounding the liquid crystal cell. An electronic control circuit is adapted to apply selectively a voltage signal to the conductive think film and the conductive mirrored coating to affect the transmittance of the liquid crystal fluid, and thereby the darkness of the mirror, in response to light intensity sensed by a headlight sensor coupled to the electronic control circuit.

REFERENCE TO RELATED APPLICATION

Priority is hereby claimed to the filing date of U.S. provisional patentapplication Ser. No. 61/085,921 filed on Aug. 4, 2008, and the entirecontents of that provisional patent application are incorporated byreference.

BACKGROUND

For driving safety and comfort, it has long been necessary to controlthe reflectance of rearview mirrors in automobiles and other motorvehicles. At the driver's preference, the mirror reflectance can bealtered from a highly reflective or “bright-mode” (BM) state to a lessreflective “dark-mode” (DM) state. When other vehicles with brightheadlights approach from behind at night, the rearview mirror isswitched from its BM state to its DM state to minimize the headlightglare reflection from the rearview mirror since the annoying lightintensity can hinder the driver's efforts to maneuver the vehiclesafely. Conventionally, such adjustment was controlled manually by amechanical lever that tilts the mirror reflecting angle slightly awayfrom the driver. Later, the mechanical lever was controlled byelectromechanical means, but still was done on demand by the driver.More recently, rearview mirrors are commercially available thatautomatically vary the reflectance of a mirror from BM to DM or viceversa using electrochromic (EC) tinting technology without having tomove the mirror physically.

An EC mirror dims when a Direct Current (DC) is applied between thefront and back electrodes of a mirror cell that contains a chemicalelectrolyte. The dimming mechanism is an electrochemical plattingprocess that proceeds slowly. An EC mirror generally takes 6 to 10seconds to change from its original BM to a less reflective deep greencolor in its DM. Due to the complex electrochemical platting processeswith the EC technology, the response time depends critically upon mirrorsize. The larger the mirror's area, the longer it takes to alter thereflectance of the mirror. As a result, it is not practical to build alarge EC mirror for applications such as commercial trucks. Furthermore,the electrical current requirement to change reflectance states of an ECmirror varies from 80 to 120 milliamperes. Due to this relatively highcurrent consumption, EC mirrors in automotive applications must be wireddirectly to the main power source of the vehicle; i.e., the battery.Although deriving the power through the vehicle's battery may beacceptable for an Original Equipment Manufacturing (OEM) item, it is notviable for after-market mirrors where customers demand that the mirrorbe a self-sustained unit without any requirement of external wiring.

A need persists for an automatic darkening and glare reducing mirrorthat successfully addresses these and other shortcomings of the priorart. It is to the provision of such a mirror that the present inventionis primarily directed.

SUMMARY

This invention relates generally to a glare reducing motor vehiclemirror that is constructed with a Liquid Crystal (LC) cell akin totechnology used in a Liquid Crystal Display (LCD). The LC mirror cell iscomposed of two slightly spaced apart panel substrates, which preferablyare made of glass. The front or exposed panel substrate is clear andtransparent while the back panel substrate is opaque and reflective ortransflective to form a mirrored surface instead of another transparentsubstrate as found in a regular LCD. A transflective mirror materialallowing some light to pass through is desired when displays such astemperature or compass displays are installed behind the mirror. A LCfluid with a dichroic dye mixture is enclosed in the mirror cell betweenthe front and back panels for affecting the light reflectance of themirror. More specifically, light must traverse the LC fluid with itsdichroic dye mixture as it passes through the front panel and isreflected by the back panel. If the molecules in the LC fluid areoriented parallel to the mirror surface, then the fluid is lesstransparent and the mirror is darkened. Conversely, if the molecules areoriented perpendicular to the mirror surface, then the fluid is moretransparent and the mirror is lightened. A BM embodiment is normally inits bright-mode when no power is applied and a DM embodiment is normallyin its dark mode. An Ultraviolet (UV) protective coating of inorganic ororganic material, or an UV protective polymer film on the exposedsurface of the front panel prevents the LC-dye mixture from degradingdue to extended exposure of the mirror to sunlight. An antireflective(AR) optical thin film coating is deposited atop the UV coating or theUV polymer film on the front panel to reduce unwanted reflection fromthe exposed surface of the mirror.

The LC mirror is a field-effect device that operates under the influenceof an electrical field. When coupled with and controlled by acustom-designed light sensing electronic control system, which also is apart of the invention, the LC mirror can switch automatically and veryquickly from a highly reflective BM to a less reflective DM, or viceversa. The LC mirror cell can be fabricated with glass, plastics, or anytransparent substrate that can be made into a mirror. Also, the LCmirror can be made flat or curved with a neutral black, blue, or otherDM color tint. For automotive applications, the LC mirror can beconnected to and operated on the main power source already available ina vehicle. However, due to its extremely low power consumption, the LCmirror also can be configured as a self-sustained after market clip-onmirror powered by internal dry cells, rechargeable batteries of thesame, or solar cells. Furthermore, the LC mirror can have built-intemperature and/or compass and/or clock and/or bluetooth and/or garageopener and/or tire pressure indicator and/or other types of graphicdisplays or indicators, as well as integrated backup camera displays andthe like. To eliminate power consumption when not in use, a miniatureswitch can be incorporated to turn the power on or off as needed. Amicro switch can be installed in the clip-on embodiment so that power isnot applied to the mirror system until it is clipped onto an existingmirror in a vehicle. For further energy savings, the power input to theClip-On LC mirror also can be monitored using a motion or pressuredetector to interrupt the power when no operator is on the driver's seatand/or the vehicle is not in motion.

The electric current requirement of the LC mirror, which is directlyrelated to its power consumption, is in micro-amperes, at least threeorders of magnitude less than that of a prior art EC mirror. The timerequired for the LC mirror to change states from BM to DM is inmilliseconds, three orders of magnitude faster than time required for aprior art EC mirror to change states. There is no color shift for the LCmirror as it changes from BM to a less reflective neutral black DM orvice versa. Prior art EC mirrors, by contrast, shift through a colorband during the several seconds required to change states beforesettling at a deep green color tint. Also, mirror size is not a limitingfactor for the LC technology of the present invention. Further, a BM LCmirror can revert quickly to BM if the power is removed suddenly, whilea prior art EC mirror takes more than 10 seconds to switch when thepower is completely lost for whatever reason. As a public safetyconcern, the U.S. Department of Transportation (DOT) has determined thatan electrical dimming mirror for motor vehicles must quickly andautomatically revert to a BM state in case of power failure, which isreadily accomplished with the BM mirror embodiment of the presentinvention. Because the LC mirror is an electrical field effect device,it can stay in its DM state practically for an indefinite length of timewithout degradation in performance.

Thus, an improved dimming automotive mirror is now provided that, amongother advantages, switches from its BM to its DM or vice versa virtuallyinstantaneously, that consumes very little electrical power and thus canbe operated on internal batteries, and that is not limited to smallsurface area mirrors. The invention will be better understood uponreview of the detailed description set forth below taken in conjunctionwith the accompanying drawing figures, which are briefly described asfollows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section showing one configuration of an LC mirroraccording to principles of the invention.

FIG. 2 illustrates molecule orientation in a Dark-Mode or DM mirror cellfilled with LC fluid and a dichroic dye mixture.

FIG. 3 illustrates molecule orientation in a Bright-Mode or BM mirrorcell filled with LC fluid and a dichroic dye mixture.

FIG. 4 is a schematic block diagram of the LC mirror electronics.

FIG. 5 is a schematic block diagram of an LC mirror control circuitaccording to the invention.

FIG. 6 illustrates one configuration of a Clip-On LC mirror.

FIG. 7 illustrates optional features for an OEM LC mirror.

FIG. 8 illustrates optional features for a Clip-On LC mirror.

DETAILED DESCRIPTION

Prior to a detailed discussion of the invention with reference to thefigures, several aspects of the mirror system of the invention and ofits electronic control system will be described to clarify the detaileddescription that follows. These aspects include the various componentsof the LC chemical system, and the mirror control electronics.

1. LC Chemical System

The performance of the LC mirror of the invention depends on thechemical system, which includes the nature and properties of thealignment compound for the LC molecules and its closely related to theLC material formulation. The specific LC fluid with dichroic dyestuffsworks in harmony with the alignment compound that has been developed.These two materials must be compatible and incorporate well to establishthe required LC molecule orientation for maximum optical performance.With the LC technology described herein, two types of LC mirrors can beconstructed. The first type is a DM mirror that normally is at itslowest reflecting DM state and changes to its highest reflecting BMstate only when an alternating current (AC) voltage signal is appliedacross the LC fluid. The second type is a BM mirror that normally is atits highest reflecting BM reflective state and changes to the lowestreflecting DM state only when an AC voltage signal is applied across theLC fluid.

1.1 Alignment Compound

The LC molecule orientation is different with the two different mirrortypes. In the DM mirror, the normal orientation of the LC molecules,i.e. their orientations without a voltage signal applied, is in ahomogeneous configuration with the molecules aligning substantiallyparallel to mirror substrates. In contrast, the normal orientation ofthe LC molecules in the BM mirror is in a homeotropic configuration withthe molecules aligning substantially perpendicular to the mirrorsubstrates.

For the DM mirror, the homogeneous alignment compound may be but is notlimited to Methyl Cellulose, Polyvinyl Alcohol, or Polyimide (PI)materials. In conventional LCD manufacturing, the alignment compound iscomposed of 1-6% PI in N-Methyl Pyroridone (NMP) or other appropriateorganic solvent. The alignment compound is usually applied onto thesurfaces of the glass substrates that bound the gap by roller coating orscreen printing methods.

The LC molecules in the BM mirror normally are in a homeotropicorientation substantially perpendicular to the cell walls, which is nota common orientation for a traditional LCD product. The homeotropicalignment can be established by materials including, but not limited to,Quilon chrome complexes by Du Pont or sputter coating of a silicon-oxidethin film. In the current invention, organic compounds with aliphaticlinear carbon side chains or chains of but not limited to C4-C30 areused as the alignment compound at a concentration of, but not limitedto, 1-6% solution in a solvent such as Isopropyl Alcohol (IPA),Propanediol, Propylene Glycol Monoethyl Ether Acetate, or otherappropriate organic solvent. The alignment material of this invention isplanted and anchored onto the mirror cell surfaces instead of forming apolymer network layer as in the conventional LCD alignment method. Thealignment compound can be formulated for various application techniquesincluding dipping, flowing, screen printing, roller coating, off-setprinting, or other techniques.

1.2 Liquid Crystal Fluid

For the DM mirror embodiment of the invention, wide temperature range(−40 to +120° C.) LC compounds of but not limited to static drive fluidswith drive voltage from 1.5 to 5.0 V AC can be used as startingcomponents for the LC fluid formulation. These materials arecommercially available from EM Industries or other LC producersworldwide. However, it may be necessary to fine tune the materialformulations for specific applications.

For the BM mirror, LC compounds with a negative dielectric anisotropy Δ∈of −3 to −6 or lower are preferred. This negative dielectric LC materialis needed for the LC molecules to rotate and align themselves from theirnormally perpendicular orientations to parallel orientations when an ACvoltage signal is applied across the front and back electrodes of themirror cell creating an electric field. The higher the absolute value ofthe negative dielectric anisotropy constant Δ∈, the lower the drivevoltage needed to change the mirror reflectance.

1.3 Dichroic Dye

The dichroic dyestuffs are high molecular weight organic compounds withrod shape molecular structures. Red, blue and yellow dyes are threebasic colors in the present dye formulations. Other colors such aspurple, brown, pink, etc. also may be used. Azo and light stableanthraquinone dyes with high solubility in the LC compounds are mostuseful for application in the present invention. The order parameterthat defines the orientation of the dye molecules against the LCmolecules is preferred to be in the range of 0.7-0.9. The higher theorder parameter number, the better the contrast ratio for the LC mirror.It must be noted that the order parameter is specific to the LC-dye pairand must be established by empirical measurements.

The individual dye composition in the LC formulation varies from 0.1 to4.5%, depending on the color requirement including, but not limited to,black and other colors. The overall dye composition in the LCformulation for DM or BM mirror is 0.8 to 4.5% as determined by therequired LC types and color saturation. If dye composition isoversaturated in the dichroic LC fluid, dyes can recrystallize out,which can cause undesirable results.

2. Mirror Control Electronics

The LC mirror control electronics comprise a printed circuit board (PCB)containing but not limited to two photo detectors and other componentsas needed for monitoring ambient light, headlight intensity, and forcontrolling the mirror reflectance in automatic response. The firstphoto detector is an ambient light sensor (AS), which is aimed in thetravel direction of the vehicle or any other direction that is notrearward. The AS is in control during daytime to prevent the mirror fromdimming in daylight. The second is a headlight sensor (HS) directedtoward the back of the vehicle to monitor headlight intensity of atrailing vehicle at nighttime. When the ambient light detected by the ASfalls below a preset level, indicating darkening ambient conditions, theHS becomes the controlling sensor to alter the reflectance of the mirrorin response to bright headlights from behind that exceed a presetintensity level. The mirror control electronics can be powered byconnecting the LC mirror to the existing main power source of a vehiclefor OEM mirrors, or can be powered by internal batteries.

As mentioned, the mirror control electronics also can be powered bytypical electronic instrument or equipment type batteries or cells, orrechargeable batteries, independent of the main power source of thevehicle. This allows the LC mirror to be built into a self-sustaineddevice feasible for after-market consumer application. A dry-cellbattery operated LC mirror can be readily clipped onto an existinginterior rearview mirror of a vehicle. With proper tooling, exteriormirrors also can be readily built into similar clip-on configurationsfor various model vehicles. With an OEM application, power consumptionis of no concern since the power comes from the existing source of thevehicle. For a Clip-On LC mirror using dry cell batteries, powerconsumption becomes an important factor.

Referring now in more detail to the drawing figures, FIG. 1 shows, incross section, the configuration of a LC mirror that embodies principlesof the invention in a preferred form. The LC mirror is constructed withtwo flat (or curved) substrates such as glass or plastic panels. Themirror front substrate 101 is a clear or transparent panel depositedwith a transparent electrical conductive thin film 102, including, butnot limited to, an Indium-Tin Oxide (ITO) coating. An organic alignmentcoating 103 for LC alignment is deposited over the conductive thin film102 as shown. An UV protection coating or a polymer film 104 isdeposited or laminated on the opposite or exposed surface of thesubstrate 101. This UV coating may comprise an inorganic optical thinfilm or an organic coating formulated with UV absorption agents such as,but not limited to, Tinuvin compounds by Ciba Specialty ChemicalsCorporation or Uvinul materials available from BASF Corporation, in aproper polymer resin as the base of the coating. The UV protectivepolymer film can be but is not limited to the products of Nanofilm Co.,Ltd., Korea. An AR optical thin film 105 of, but not limited to, aninorganic type is deposited over the UV coating 104 to reduce the frontsurface reflectance of the substrate 101. Such AR coatings arecommercially available and adapted to the present application. Also, anUV protective polymer film with an AR topcoat is currently availablefrom Nanofilm.

The back substrate 109 may be the same as the front substrate 101, butwith a highly reflective or transflective thin film 108, including butnot limited to a hybrid coating of ITO and/or silicon-oxide and/ortitanium-oxide silver or an enhanced aluminum thin film. The LCalignment coating 107, which is the same as coating 103, is depositedover the mirrored surface 108. A mirror cell gap is defined between thefront and back substrates. A LC fluid 106, which carries light screeningdichroic dyes according to the invention, is contained in the mirrorcell between the front 103 and back 107 alignment coatings applied onthe surfaces of the substrates. The mirror cell gap is maintained byspacers 110 in the range of, but not limited to, 3-12 microns and sealedby adhesive seal 111.

FIG. 2 is a cross section of a cell portion of a dichroic LC DM mirrorshowing alignment coatings 203 on the front substrate and 207 on theback substrate, LC fluid 206, and representations of LC host molecules204 and guest dye molecules 205. Adjacent to the mirror front substrate,the LC host molecules 204 orientate themselves generally parallel to thesurface of the alignment coating 203, which has been mechanically buffedin a defined direction. The guest dichroic dye molecules 205 take apiggyback ride position with the host LC molecules 204. A similaralignment relationship but with a perpendicular orientation relative tomolecules adjacent to the front substrate is established for the LC anddye molecules 209 and 210 at the mirror back alignment coating 207,which is mechanically buffed in a direction 90 degrees clockwise orcounter clockwise relative to the top alignment coating 203 depending onthe viewing characteristics as desired. With this alignmentconfiguration, the LC mirror is normally at a DM state when notactivated because the combined light blocking area presented by the dyemolecules in a direction parallel to the mirror surfaces issubstantially maximized.

FIG. 3 is a cross section of a similar cell portion of a dichroic LC BMmirror according to an alternate embodiment. In this embodiment, the LChost molecules 224 and dye molecules 225 align themselves substantiallyperpendicular to the wall surfaces as directed by the specific alignmentcoating 221 on the front and 222 on the back of the mirror cell. Withthis alignment configuration, the LC mirror is normally at a BM statewhen not activated as the light blocking area presented by the dyemolecules in a direction perpendicular to the mirror is substantiallyminimized.

FIG. 4 is a functional block diagram of an LC mirror electronic controlsystem according to the invention. As discussed above, the reflectanceof the LC mirror is determined by the orientation of the LC and dyemolecules 204 and 205 in FIG. 2 for the DM mirror and 224 and 225 inFIG. 3 for the BM mirror. The mirror reflectance is changed by alteringthe phase relationship and strength of a square wave voltage signalapplied across the inside front transparent conductive coating electrode102 and the back reflective or transflective surface electrode 108 ofthe mirror cell in FIG. 1. As the voltages across the front 102 and back108 electrodes in FIG. 1 are in phase, the mirror is in its mostreflective BM state. When the front and back voltages are π radians or180 degrees out of phase, the mirror is at its least reflective stateDM. The mirror reflectance increases as the out-of-phase voltage isreduced toward an in-phase state.

A control strategy and system is required to determine when, the amount,and the phase of the voltage applied to the mirror electrodes that causethe mirror to become less reflective and thus reduce the glarereflection from the headlights of a trailing vehicle. The detectionsystem should first determine if the ambient lighting indicates either adaytime or nighttime condition. Then the system should determine, atnight only, when vehicles with bright headlights are approaching frombehind and whether the incident light from the vehicle's headlights issufficient to interfere with the vision of the driver. Additionalenhancements to these two conditions and the control strategy also haveshown to improve the LC mirror operation.

Referring to FIG. 4, the photo detector system of the preferredembodiment is operated according to the following logic. When theambient light sensor (AS) 301 detects a light level higher than a presetthreshold value, the headlight sensor (HS) 302 is disabled and themirror remains in its bright mode state for either the DM or BM mirror.HS 302 is enabled when AS 301 detects a light level equal to or lessthan the preset threshold value, indicating that it is approachingnighttime, nighttime, or a low ambient light level equivalent tonighttime. Then, when HS 302 detects a headlight intensity equal to orhigher than a preset threshold level, the mirror control circuit 303will dim the mirror 304 to its least reflective DM state and return itto its BM state as the headlight brightness is reduced to lower than thepreset threshold level. The light level or threshold of HS 302preferably is preset at the same level as that of the AS 301 to reducethe length of dusk or dawn time period that usually lasts approximately15 to 30 minutes depending on seasons of the year, the latitude, and thedirection of travel. On the other hand, during the dusk or dawn timeperiod, the mirror will stay at DM if the preset light level of HS 302is lower than that of AS 301 as the ambient light will turn the mirrorto the DM state.

There are criteria for the photo sensing system, i.e. the ambient andheadlight sensors, of the present invention, particularly regardingsetting the threshold level for the ambient light sensor 301. If thesensitivity of AS 301 is set too low (i.e. it responds to a brighter ormore intense lighting condition), the mirror will be dimmed prematurelybefore it is sufficiently dark outside. If it is set too high (i.e. itresponds to a darker or less intense lighting condition), the mirrorwill not begin to darken in response to headlights from behind untillater in the evening when it becomes very dark. The preferred settingfor the ambient light sensor should be a light intensity levelindicative of approximately 15 minutes before sunset when most motorvehicles have their headlights turned on. In the present invention, thelight sensing function primarily is controlled by the ambient lightsensor. If the sensitivity of AS 301 is set too high and the sensitivityof HS 302 is set even higher than that of AS 301, the mirror will notdim on bright headlights until the ambient light level is equal to orlower than the preset level of AS 301. In such instances, the mirrorwill not function as darkness approaches. If the sensitivity of HS 302is set lower than that of AS 301, the mirror will start to dimprematurely before sunset. It has been found that the best scenario isto set the sensitivity of HS 302 approximately the same as that of AS301. With both AS 301 and HS 302 set at about the same light level, themirror will dim in response to headlights from behind as soon as AS 301enables HS 302 at the preset ambient light threshold, preferred to beindicative of about 15 minutes before sunset. As darkness deepens andthe ambient light level becomes lower than the preset level of HS 302,the mirror will start to function normally. There may be a short timeperiod of several minutes during which the mirror may be darkened due toambient light instead of headlights from behind. To minimize the effectsof the dusk/dawn transitions with the ambient sensor, a slighthysteresis is included in the sensor comparator circuits.

When a vehicle goes under highway bridges or through shaded areas duringdaytime under low overcast conditions, AS 301 can enable HS 302 which inturn may be triggered by the ambient light level to dim the mirrorirregularly or intermittently depending on the speed of the vehicle.Using low power consumption capacitors for energy conservation, a timedelay (TD) function 305 is built into the control circuit 303 for theambient sensor to establish a 1-4 second time delay before the mirrorswitches from DM to BM to reduce irregular dimming or flickering of themirror. A similar TD function 306 also is built into headlight sensor302 to prevent the mirror from flickering when headlights of anapproaching vehicle do not cast a steady light, such as when driving onuneven pavements when followed by a bright headlight or high beam. TheTD feature of the ambient sensor is to reduce flickering of the mirrorduring daytime driving in low overcast environments and that of theheadlight sensor is to minimize flickering when driving on unevenpavements and/or under heavy traffic conditions. Also, the comparatorhysteresis minimizes the flicker effect.

Details of electronic control schemes vary slightly for the DM and BMmirror embodiments, the primary differences being summarized as follows:

Dark-Mode Mirror

A DM mirror is at its lowest reflective state when not activated, i.e.when no control voltage is applied to the electrodes. The mirrorswitches to BM only when the AS 301 enables the HS 302, and only when HS302 detects a light level higher than its preset threshold, which causesthe mirror control circuit 303 to switch the mirror 304. As controlboard 303 is enabled, it generates an out-of-phase square wave ACvoltage across the front and back electrodes to alter the reflectivemode of the mirror 304 from its original DM to the more reflective BMstate.

When the ambient light level is below the preset threshold of AS 301,control circuit 303 enables HS 302. As HS 302 is enabled, but the lightlevel of trailing headlights is nevertheless below the preset level ofHS 302, control circuit 303 is enabled and the mirror 304 is switched toits most reflective BM state. When HS 302 is enabled and the light levelis above its preset level, control circuit 303 disables the mirror 304and returns it to the original lowest reflective DM state. It will thusbe seen that the glare reflection of the mirror is reduced as a vehicleapproaches from behind at night with bright headlights or high beams.

Bright-Mode Mirror

A BM mirror is at its highest reflective state when not activated. Themirror switches to DM only when AS 301 enables HS 302 and only when HS302 detects a light level higher than its preset threshold, which causesthe mirror control circuit 303 to switch the mirror 304. Morespecifically, the control circuit 303 generates an out-of-phase squarewave voltage across the front and back electrodes of the LC cell toalter the reflective mode of the mirror 304 from its original BM to theless reflective DM state.

When the ambient light level is below the minimum threshold of AS 301,control circuit 303 enables or transfers control to HS 302. As HS 302 isenabled and the light level from headlights of a trailing vehicle isabove the HS 302 preset threshold, control circuit 303 applies the ACvoltage and the mirror 304 is switched to its less reflective DM state.When HS 302 is enabled and the light level of trailing headlights isbelow its preset threshold, control circuit 303 disables the mirror 304by discontinuing the voltages applied to the electrodes, and returns themirror to the original more reflective BM state. It will thus be seenthat the glare reflection of the mirror is reduced as a vehicleapproaches from behind with bright headlights or high beams.

FIG. 5 is an LC mirror control circuit functional schematic blockdiagram according to the invention. The two amplifier/peak detectors3011 and 3021 have output voltages that are directly proportional to theincident light levels impinging upon the AS 301 and the HS 302,respectively. These output voltages, rapid variations or “flickering” ofwhich are effectively filtered out by time delays 306 and 305, areadmitted to voltage comparators 3012, and 3022, respectively. The biasvoltage on the ambient comparator 3012 is set on the positive inputslightly higher than the voltage output of amplifier/peak detector 3011for dark conditions. In this way, the output voltage of the ambientcomparator 3012 is at a logic level high when there is no orinsufficient light incident upon AS 301, and the output of the ambientcomparator 3012 is a logic level low when light of sufficient intensityis incident upon AS 301. A time delay function 3051 of 2-10 seconds isimplemented to reduce flickering of the mirror during times when theambient light level is at or close to the preset threshold, which cancause rapid fluctuations in the output of the amplifier/peak detector3011.

Similarly, the bias voltage on the headlight comparator 3022, is set onthe negative input slightly higher than the output voltage ofamplifier/peak detector 3021. Thus, the output of the headlightcomparator 3022 is at a logic level low when there is no or insufficientlight incident upon headlight sensor 302 and the output of the headlightcomparator 3022 is logic level high when sufficient light is incidentupon headlight sensor 302. A similar time delay function 3052 of 2-10seconds is added to reduce flickering of the mirror during times whenthe intensity level of trailing headlights is close to the presetthreshold.

The outputs of comparators 3012 and 3022 are the inputs to a two inputNAND gate circuit 3031. The output of this NAND gate circuit 3031 is lowonly when both inputs are in a logic level high, and is high when eitheror both comparator outputs are at logic low. The only condition in whichboth comparator outputs are high is when AS 301 and amplifier/peakdetector 3011 indicate ambient lighting conditions are dark, i.e. thevoltage signal of AS 301 is below the setting of the peak detector, andHS 302 and its amplifier/peak detector 3021 indicate that there areheadlights from behind with an intensity greater than a predeterminedthreshold, i.e. the voltage signal of HS 302 is above the setting of thepeak detector.

The output of the NAND circuit 3031, is inverted by a NAND gateconfigured as a logic inverter 3032, such that when both comparator3012, and 3022 outputs are high, the output of the logic inverter 3032is high.

Two additional NAND gates are configured as a square wave oscillator3033, with an output frequency set to drive the liquid crystal mirror304 to lower the reflectivity of the mirror and not appear as a flicker.Typically, this frequency is from about 30 to about 60 Hz, althoughother frequencies are possible and within the scope of the invention.The oscillator 3033 is in operation only when the output of the inverter3032 is high and the oscillator 3033 is low or off when the logicinverter 3032 output is low. It will thus be recognized that with thecircuit of FIG. 5, a square wave voltage signal is applied to the mirroronly when the ambient sensor indicates sufficient ambient darkness andthe headlight sensor indicates sufficient brightness from behind.Oscillator operation is given in the truth table below with typicalsupply current requirements for the four conditions.

Ambient Headlight Oscillator Current Sensor Sensor Conditionmicro-amperes Daylight Low Low Off 30 No Headlight Daylight Low High Off40 with Headlight Night High Low Off 40 No Headlight Night High High On250 with Headlight

Some or all of the electronic functions described herein can beperformed by a microprocessor, a FPGA (field programmable gate array),an ASIC (application specific integrated circuit), or other type ofelectronic circuit. The embodiment described is for a two-stateoperation between either maximum reflection or reduced reflection toreduce glare. However, the amount of reflection and glare reduction alsocan be controlled between these two discrete levels by varying the dutycycle of the applied alternating voltage signals to the mirrorelectrodes. This pulse width modulation to control the degree ofreflectivity can then be a direct function of the conditions measured bythe light sensors. Also, the time in which the ambient sensor is indawn/dusk transition can be controlled by a timer or clock when thatfunction is included in the display section of the mirror.

FIG. 6 shows one embodiment of a mounting mechanism for mounting aClip-On embodiment of the LC mirror 401 to and existing rearview mirror409. Movable arms or clips 402 and 403 slide up and down in corporationwith the stationary arms 404 and 405 for clamping the LC mirror 401 ontothe existing mirror 409 of a vehicle. The movable mounting arms or clips402 and 403 preferably are biased to their retracted positions bysprings inside the case of the mirror 401. Although in the presentdemonstration, the two movable arms are located at the bottom of themirror case, they can be positioned either on the top or at the bottomof the mirror case. To mount mirror 401 onto mirror 409, the movablearms are extended, the fixed arms 404 and 405 are positioned atop (or onbottom) the existing mirror, and the movable arms released causing themto retract partially and thus clamp onto the bottom (or top) of themirror 409

The batteries 408, which are inside the case of the mirror 401, can beconnected and disconnected automatically from the circuit by a miniatureswitch 407 that detects when the mirror 401 is or is not attached tomirror 409. For example, the switch 407 can be configured to be openwhen arm 402 or 403 is in its fully retracted position, meaning that themirror 401 is not attached to existing mirror 409, and closed when arm402 or 403 is extended, indicating that the mirror 401 is attached tomirror 409. As the mirror 401 is attached to the existing mirror 409,switch 407 applies battery power to the control board 406. In this way,battery power is not depleted when the mirror 401 is not in use duringstorage or transportation. Also, the TD function is incorporated intothe mirror control circuit 406 using low power consumption capacitorsfor energy conservation.

The Clip-On LC mirror 401 typically operates at a square wave voltage of4.5 volts or greater at a standby mode of approximately 50 micro-amperesand an operational mode at 200 micro-amperes using typically 3 eachalkaline type 1.5 volt batteries in series to provide a supply of 4.5volts nominal DC output. With a custom designed wire harness, theClip-on LC mirror can be connected directly to the existing car batterythrough a power outlet such as the cigarette lighter. Also, the powersource can be rechargeable type batteries or solar cells.

The DM type LC mirror can require substantially more energy to operateover time than the BM mirror because the DM mirror must be at itshighest reflective BM state most of the time and this state is obtainedin the DM mirror by applying an AC voltage signal to the mirror'selectrodes. As a result, the DM mirror may be more effective for OEM usewhere power consumption is not a critical issue because the mirror ispowered by the car battery. The BM mirror likely will be most efficientfor after-market clip-on applications since it draws power from itsbatteries only at its DM state, which occurs much more rarely than theBM state.

FIG. 7 depicts possible added features that may be incorporated in anOEM LC mirror 501 for but not limited to Temperature 502, Compass 503,and Graphic Display 504. Other features such as clock, bluetooth, garageopener, tire pressure indicator, etc. also can be incorporated in themirror, if desired. The temperature function may include interior andexterior temperatures. The graphic display 504 may present a back-upcamera view, a message center, and/or GPS information.

FIG. 8 presents possible added features for an after-market Clip-On LCmirror 601 for but not limited to Temperature 602 and Compass 603. Otherlow power consumption features such as clock, bluetooth, garage opener,tire pressure indicator, etc. also can be incorporated in the mirror, ifdesired. A miniature manual power switch 604 and a Motion Detectorswitch 605 are additional energy saving devices for the Clip-On LCmirror since the battery 606 can be disconnected manually through thepower switch or automatically by the motion detector as the dimmingfunction is not needed when a vehicle is not in motion.

As described above, the ambient light sensor determines daylightconditions from night conditions. The headlight sensor is in the activemode only when the ambient sensor detects darkness or an otherwise lowlight condition. This functionality is achieved through the logiccircuitry as described above. If the headlight sensor then detects abright headlight behind the vehicle, the oscillator will apply an ACvoltage to the electrodes of the LC mirror to make the mirror lessreflective and thereby to reduce the glare from the trailing vehicleheadlights. Rather than having a simple on/off function for theoscillator, the voltage level detected by the headlight sensor also canvary the duty cycle of the oscillator so the degree of reflectivity orglare reduction can be more for brighter lights and less for lights thatare less brilliant. As a result, the mirror can be dimmed gradually todifferent reflection levels automatically responding to the lightdetected by the headlight sensor. This feature has been successfullydemonstrated in the LCD mirror of the present invention but is notpossible in prior art electrochromic dimming mirrors.

Additionally, as also discussed, the comparator that determines theactivation level for the ambient sensor has a time delay of a fewseconds so that the circuit does not vacillate during the transitionfrom light to darkness or from darkness to light at dusk and dawn times.An alternative technique for eliminating this unstable transition is toinclude a hysteresis in the comparator, so that the level from low tohigh is lower than that from high to low. A transition gap is built intothe electronics to minimize excessive on/off of the ambient state.

Various prototype LC mirrors in both the OEM and Clip-On configurationshave been constructed and tested by the inventors. The size of theprototypes varies from mirrors for small cars up to seven inch by 16inch mirrors for long haul trucks. Prototypes in both DM and the BMconfigurations have been tested and demonstrated to be successful.Mirror systems have been built and proven to be capable of switching themirrors back and forth from bright to transparent neutral black or bluecolor. These prototypes have been proven to be viable review mirrorproducts in OEM and after-market applications.

Various features, aspects, additions, deletions, and modifications ofthe forgoing disclosure may be recognized by skilled artisans within thescope of the invention. Some possible examples are listed below.

1. The LC mirror cell is composed of a transparent front and areflective or transflective back substrates made of material includingbut not limited to glass and plastics.

2. The bottom surface of the front substrate of the LC mirror in 1 isdeposited with a transparent conductive thin film made of materialincluding but not limited to Indium-Tin Oxide (ITO) or Tin-Oxide.

3. The top surface of the LC mirror front substrate in I is depositedwith a UV protection coating including but not limited to an inorganicoptical thin film made of multilayer oxide materials or laminated with aUV protective polymer film.

4. The UV protective coating in 3 also can be made of an organic coatingformulated with materials including but not limited to the UV absorptionreagents such as Tinuvin compounds by Ciba Specialty ChemicalsCorporation or Uvinul materials by BASF Corporation. The content of theUV absorption reagents is 2-15% in a polymer base carrier.

5. An inorganic type AR optical coating including but not limited to thehybrid oxide thin films is deposited on the top surface of the UVprotection coating in 3 and 4 for cutting the light reflectance at andabove 410 nm from the front surface of the LC mirror cell to reducevisible light reflection.

6. The UV protective polymer film in 3 can be but is not limited to theproducts by Nanofilm Co. of Korea. Also film products incorporated withan AR topcoat also are available from the same supplier.

7. The top surface of the back substrate of the LC mirror in 1 isdeposited with a high reflective or transflective thin film made ofmaterial including but not limited to hybrid thin films of ITO,silicon-oxides, titanium-oxides silver, aluminum, enhanced aluminum,chrome, or other reflective or transflective coatings.

8. The mirror surface in 7 is protected with a transparent topcoatincluding but not limited to non-conductive Indium-Oxide,Titanium-Oxide, Titanium-Nitride, or other passivation thin films toprotect the reflective coating from degradation.

9. The surface with the transparent conductive thin film in 2 and thesurface with the reflective coating in 8 are coated with an alignmentmaterial for establishing the LC molecular orientations.

10. The homogeneous alignment for the LC molecules of the mirror cell in1 is established by the material including but not limited to the PI,Polyvinyl Alcohol, or other appropriate organic compounds.

11. The homeotropic alignment for the LC molecules of the mirror cell in1 is established by the material including but not limited to organiccompounds with straight carbon side chains or chains of C4-C30 inlength.

12. The mirror cell gap in 1 including but not limit to 3-12 microns inthickness.

13. The LC mirror cell in 1 is filled with a dichroic LC fluid.

14. For Dark-Mode mirror application, the dichroic LC fluid in 10includes a dichroic dye mixture dissolved in a LC mixture with apositive dielectric anisotropy constant Δ∈.

15. For Bright-Mode mirror application, the dichroic LC fluid in 11includes a dichroic dye mixture dissolved in a LC material with anegative dielectric anisotropy constant Δ∈.

16. The dichroic LC fluids of 14 and 15 are composed of three base colordyes including but not limit to red, blue, and yellow. The overall dyecomposition of the LC fluids in 14 and 15 is but not limit to 0.8 to4.5% with 0.1 to 4.0% for each individual dye component.

17. The order parameters of the dye-LC molecule pair in 14 and 15 arebetween but not limited to 0.7-0.9.

18. The LC mirror in 1 assembled with 2 through 17 is in a flatconfiguration.

19. The LC mirror in 1 assembled with 2 through 17 also can be in acurved configuration.

20. The LC mirror in 18 can be made in black Dark-Mode or Bright-Modetype.

21. The LC mirror in 18 can be made in blue Dark-Mode or Bright-Modetype.

22. The LC mirror in 19 can be made in black Dark-Mode or Bright-Modetype.

23. The LC mirror in 19 can be made in blue Dark-Mode or Bright-Modetype.

24. The light reflectance of the LC mirrors in 18 through 23 ismonitored automatically by electronic means.

25. The electronic mirror control system in 24 comprises an ambientlight sensor incorporated with a headlight sensor.

26. The ambient light sensor in 25 is designed to inhibit the reducedreflection feature during daylight hours of operation.

27. The ambient light sensor in 25 is equipped with an optical filter tomaximize its sensitivity toward the ambient daylight and minimize itssensitivity toward the artificial headlights of on-coming traffic.

28. The sensitivity of the ambient sensor in 26 and 27 is set tocorrespond to ambient light levels at about but not limit to 15 minutesprior to sunset.

29. The sensitivity of the headlight sensor in 25 is set to besubstantially the same as that of the ambient sensor in 26 and 27.

30. A time delay of but not limit to 2-6 second using resistor circuitis incorporated into the control logic of 28 for the ambient sensor tominimize mirror flickering during low overcast ambient environmentduring daytime and driving on uneven pavements or in heavy trafficconditions at night or in low light conditions, also at dusk or dawntime when the ambient light level may overlap with the headlight level.

31. The headlight sensor in 29 is designed to minimize its sensitivitytoward the ambient daylight and maximize its sensitivity toward theartificial headlight to prevent the mirror from faults dimming indaytime.

32. A time delay of about but not limit to 2-6 second using resistorcircuit is incorporated into the control logic of 31 for the headlightsensor to inhibit the mirror from flickering at nighttime during heavytraffic conditions and when vehicles are traveling on the unevenpayments.

33. The headlight sensor in 32 activates and dims the LC mirror toreduce the glare reflection when a vehicle approaches from behind withbright headlights or high beams.

34. The LC mirror of 1 assembled with the electronic system as coveredin 2 through 33 is for production of the interior and exterior rearviewmirrors for motor vehicle manufacturers.

35. The LC mirrors in 34 is powered by the main power source in themotor vehicle.

36. With low power consumption, the LC mirrors of 35 are also designedinto a self-contained Clip-On configuration powered by dry cellbatteries. These LC mirrors can be clipped onto the existing rearviewmirrors of motor vehicles.

37. The Clip-On LC mirror in 36 is powered by alkaline or lithium typebatteries.

38. The Clip-On LC mirror in 36 also can be powered by rechargeablebatteries.

39. The Clip-On LC mirror in 36 also can be powered by solar cells usedin conjunction with a charging circuit for a rechargeable battery orsuper-capacitor.

40. The Clip-On LC mirror in 36 also can be powered by connectingdirectly to a power outlet, such as but not limit to the existingcigarette lighter in a vehicle.

41. Power supply to the Clip-On mirrors in 36 is interrupted manually byan on/off switch.

42. To conserve power consumption of the dry cell batteries, a miniatureswitch is installed into the electronic control circuit of the Clip-Onmirror in 36 and mounted with a tension spring on to a clipping arm sothat power input to the mirror is interrupted when the mirror is notclipped onto to the existing mirror in a vehicle.

43. Power supply to the Clip-On mirrors in 36 is interrupted by a motiondetector when the vehicle is in a static or parked mode but power isresumed to the mirror when the motion detector detects motion of thevehicle.

44. A low battery indicator activates when the battery input to the LCmirror of 36 is low.

45. A compass is incorporated into the LC mirrors in 35 and 36 fordirectional orientation.

46. A thermal sensor is incorporated into the LC mirrors in 35 and 36for monitoring the external temperature of a motor vehicle.

47. A thermal sensor is incorporated into the LC mirrors in the said 35and 36 for monitoring the internal temperature of a motor vehicle.

The invention has been described herein in terms of preferredembodiments and methodologies considered by the inventors to representthe best mode of carrying out the invention. It should be understood,however, that a wide variety of additions, deletions, and modificationsmight be made to the illustrated embodiments by those of skill in theart without departing from the spirit and scope of the invention as setforth in the claims.

What is claimed is:
 1. A liquid crystal mirror that darkens in responseto headlights impinging on the mirror, the liquid crystal mirrorcomprising: a substantially transparent front substrate having interiorand exterior surfaces and a substantially transparent and conductivelayer on the interior surface of the front substrate; a back substratehaving interior and exterior surfaces and being spaced from the frontsubstrate to define a liquid crystal cell between the interior surfacesof the substrates, and an at least partially reflective and conductivelayer on the interior surface of the back substrate; an alignmentcompound on opposite sides of and bounding the liquid crystal cell; aliquid crystal fluid contained within the liquid crystal cell, theliquid crystal fluid including a dichroic dye; an electronic controlcircuit coupled to the transparent conductive layer on the frontsubstrate and the reflective conductive layer on the back substrate, theelectronic control circuit adapted selectively to apply a voltage signalto the layers to affect the transmittance of the liquid crystal fluid;and a headlight sensor coupled to the electronic circuit for sensing anintensity of light impinging on the mirror, the electronic circuitapplying a voltage signal or discontinuing a voltage signal to reducethe transmittance of the liquid crystal fluid in response to a lightintensity sensed by the headlight sensor above a predetermined headlightthreshold.
 2. The liquid crystal mirror of claim 1 and furthercomprising an ambient light sensor coupled to the electronic controlcircuit for sensing an intensity of ambient light, the electroniccontrol circuit not responding to signals from the headlight sensor whenthe ambient sensor senses ambient light intensity above a predeterminedambient threshold.
 3. The liquid crystal mirror of claim 2 and whereinthe headlight threshold and the ambient threshold are substantially thesame.
 4. The liquid crystal mirror of claim 2 and wherein the electroniccircuit includes time delays applied to signals from the headlight andambient sensors to prevent rapid changes in the transmittance of theliquid crystal fluid.
 5. The liquid crystal mirror of claim 1 andwherein a mirror cell gap between the front substrate and the backsubstrate ranges from about 3 microns to about 12 microns.
 6. The liquidcrystal mirror of claim 1 and wherein the alignment compound furthercomprises an organic solvent.